Container holder utilization and selection

ABSTRACT

An inventory system includes inventory holders, container holders, containers carried by the container holders, mobile drive units, stations, and a management module. The management module instructs the mobile drive units to move the inventory holders and container holders to stations. Container holders are loaded with containers of inbound inventory items at a first station. Inbound inventory items are transferred from containers to inventory holders at a second station. Outbound inventory items are transferred from inventory holders to containers at a third station. Containers of outbound inventory items are removed from container holders at a fourth station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. application Ser. No.14/503,023, filed on Sep. 30, 2014, granted as U.S. Pat. No. 9,389,609,on Jul. 12, 2016, the entire contents of which is hereby incorporated byreference as if fully set forth herein.

BACKGROUND

Modern inventory systems, such as those in mail order warehouses, supplychain distribution centers, airport luggage systems, and custom-ordermanufacturing facilities, face significant challenges in responding torequests for inventory items. As inventory systems grow, the challengesof simultaneously completing a large number of packing, storing, andother inventory-related tasks become non-trivial. In inventory systemstasked with responding to large numbers of diverse inventory requests,inefficient utilization of system resources, including space, equipment,and manpower, can result in lower throughput, unacceptably long responsetimes, an ever-increasing backlog of unfinished tasks, and, in general,poor system performance. Additionally, expanding or reducing the size orcapabilities of many inventory systems requires significant changes toexisting infrastructure and equipment. As a result, the cost ofincremental changes to capacity or functionality may be prohibitivelyexpensive, limiting the ability of the system to accommodatefluctuations in system throughput.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments in accordance with the present disclosure will bedescribed with reference to the drawings, in which:

FIG. 1 illustrates components of an inventory system according to aparticular embodiment;

FIG. 2 illustrates in greater detail the components of an examplemanagement module that may be utilized in particular embodiments of theinventory system shown in FIG. 1;

FIGS. 3 and 4 illustrate in greater detail an example mobile drive unitthat may be utilized in particular embodiments of the inventory systemshown in FIG. 1;

FIG. 5 illustrates in greater detail an example inventory holder thatmay be utilized in particular embodiments of the inventory system shownin FIG. 1;

FIG. 6 illustrates an example of reusable containers and an associatedcontainer holder according to a particular embodiment;

FIG. 7 illustrates an inventory system that includes container holdersaccording to a particular embodiment;

FIG. 8 is a flow chart illustrating management of container holders inuse in an inventory system according to a particular embodiment;

FIG. 9 illustrates in greater detail some components that may beutilized in particular embodiments of the resource scheduling moduleshown in FIG. 2.

FIG. 10 illustrates an example scenario of determining an allocation ofinventory holders to a pick station according to an embodiment;

FIG. 11 illustrates a further example scenario of determining anallocation of inventory holders to a pick station according to anembodiment; and

FIG. 12 illustrates an example scenario of determining an allocation ofcontainer holders to discharge stations according to a particularembodiment.

DETAILED DESCRIPTION

In the following description, various embodiments will be described. Forpurposes of explanation, specific configurations and details are setforth in order to provide a thorough understanding of the embodiments.However, it will also be apparent to one skilled in the art that theembodiments may be practiced without the specific details. Furthermore,well-known features may be omitted or simplified in order not to obscurethe embodiment being described.

Embodiments herein are directed to an inventory system having multipleinventory holders and drive units for moving the inventory holders.Specifically, features herein are directed to management of reusablecontainers that are used for transporting inventory items in aninventory system, as well as techniques related to selection,organization, and management of components associated with operationsinvolving the containers.

For example, in accordance with an embodiment, a reusable containercontaining incoming inventory items, such as items from a supplier, isplaced on a portable rack at a first location to introduce the filledcontainer into the inventory system. In accordance with instructionsfrom the system, a mobile drive unit moves the portable rack, with thefilled container thereon, to a second location, where the incominginventory items are removed from the container and placed or stowed intoinventory holders, such as for storage in the inventory system, leavingthe empty container on the portable rack. The mobile drive unit thenmoves the portable rack, with the empty container thereon, to a thirdlocation where items from inventory holders are selected or picked inaccordance with instructions from the system, and placed in the emptycontainer to load the container with outgoing inventory items, such asto fulfill customer orders. The mobile drive unit then moves theportable rack, with the loaded container thereon, to a fourth location,and the container loaded with outgoing inventory items is removed fromthe portable rack and discharged from the inventory system (e.g., sentout to customers for delivery). The portable rack, now empty, returns tothe first location (or another container introduction location) toreceive more incoming containers and begin the process anew. Multipleportable racks and inventory holders are utilized in the system, and amanagement module makes determinations and issues instructions as towhich portable rack and/or inventory holder is/are to be moved at anyparticular time to any particular location to facilitate any of thedescribed operations.

Referring now to the figures, in which like reference numerals mayrepresent like parts, FIG. 1 illustrates the contents of an inventorysystem 10. Inventory system 10 includes a management module 15, one ormore mobile drive units 20, one or more inventory holders 30, and one ormore inventory stations 50. Mobile drive units 20 transport inventoryholders 30 between points within a workspace 70 in response to commandscommunicated by management module 15. Each inventory holder 30 storesone or more types of inventory items. As a result, inventory system 10is capable of moving inventory items between locations within workspace70 to facilitate the entry, processing, and/or removal of inventoryitems from inventory system 10 and the completion of other tasksinvolving inventory items.

Management module 15 assigns tasks to appropriate components ofinventory system 10 and coordinates operation of the various componentsin completing the tasks. These tasks may relate not only to the movementand processing of inventory items, but also to the management andmaintenance of the components of inventory system 10. For example,management module 15 may assign portions of workspace 70 as parkingspaces for mobile drive units 20, the scheduled recharge or replacementof mobile drive unit batteries, the storage of empty inventory holders30, or any other operations associated with the functionality supportedby inventory system 10 and its various components. Management module 15may select components of inventory system 10 to perform these tasks andcommunicate appropriate commands and/or data to the selected componentsto facilitate completion of these operations. Although shown in FIG. 1as a single, discrete component, management module 15 may representmultiple components and may represent or include portions of mobiledrive units 20 or other elements of inventory system 10. As a result,any or all of the interaction between a particular mobile drive unit 20and management module 15 that is described below may, in particularembodiments, represent peer-to-peer communication between that mobiledrive unit 20 and one or more other mobile drive units 20. The contentsand operation of an example embodiment of management module 15 arediscussed further below with respect to FIG. 2.

Mobile drive units 20 move inventory holders 30 between locations withinworkspace 70. Mobile drive units 20 may represent any devices orcomponents appropriate for use in inventory system 10 based on thecharacteristics and configuration of inventory holders 30 and/or otherelements of inventory system 10. In a particular embodiment of inventorysystem 10, mobile drive units 20 represent independent, self-powereddevices configured to freely move about workspace 70. Examples of suchinventory systems are disclosed in U.S. Patent Publication No.2012/0143427, published on Jun. 7, 2012, titled “SYSTEM AND METHOD FORPOSITIONING A MOBILE DRIVE UNIT” and U.S. Pat. No. 8,280,547, issued onOct. 2, 2012, titled “METHOD AND SYSTEM FOR TRANSPORTING INVENTORYITEMS”, the entire disclosures of which are herein incorporated byreference. In alternative embodiments, mobile drive units 20 representelements of a tracked inventory system configured to move inventoryholder 30 along tracks, rails, cables, crane system, or other guidanceor support elements traversing workspace 70. In such an embodiment,mobile drive units 20 may receive power and/or support through aconnection to the guidance elements, such as a powered rail.Additionally, in particular embodiments of inventory system 10 mobiledrive units 20 may be configured to utilize alternative conveyanceequipment to move within workspace 70 and/or between separate portionsof workspace 70. The contents and operation of an example embodiment ofa mobile drive unit 20 are discussed further below with respect to FIGS.3 and 4.

Additionally, mobile drive units 20 may be capable of communicating withmanagement module 15 to receive information identifying selectedinventory holders 30, transmit the locations of mobile drive units 20,or exchange any other suitable information to be used by managementmodule 15 or mobile drive units 20 during operation. Mobile drive units20 may communicate with management module 15 wirelessly, using wiredconnections between mobile drive units 20 and management module 15,and/or in any other appropriate manner. As one example, particularembodiments of mobile drive unit 20 may communicate with managementmodule 15 and/or with one another using 802.11, Bluetooth, or InfraredData Association (IrDA) standards, or any other appropriate wirelesscommunication protocol. As another example, in a tracked inventorysystem 10, tracks or other guidance elements upon which mobile driveunits 20 move may be wired to facilitate communication between mobiledrive units 20 and other components of inventory system 10. Furthermore,as noted above, management module 15 may include components ofindividual mobile drive units 20. Thus, for the purposes of thisdescription and the claims that follow, communication between managementmodule 15 and a particular mobile drive unit 20 may representcommunication between components of a particular mobile drive unit 20.In general, mobile drive units 20 may be powered, propelled, andcontrolled in any manner appropriate based on the configuration andcharacteristics of inventory system 10.

Inventory holders 30 store inventory items. In a particular embodiment,inventory holders 30 include multiple storage bins with each storage bincapable of holding one or more types of inventory items. Inventoryholders 30 are capable of being carried, rolled, and/or otherwise movedby mobile drive units 20. In particular embodiments, inventory holder 30may provide additional propulsion to supplement that provided by mobiledrive unit 20 when moving inventory holder 30.

Additionally, in particular embodiments, inventory items 40 may alsohang from hooks or bars (not shown) within or on inventory holder 30. Ingeneral, inventory holder 30 may store inventory items 40 in anyappropriate manner within inventory holder 30 and/or on the externalsurface of inventory holder 30.

Additionally, each inventory holder 30 may include a plurality of faces,and each bin may be accessible through one or more faces of theinventory holder 30. For example, in a particular embodiment, inventoryholder 30 includes four faces. In such an embodiment, bins located at acorner of two faces may be accessible through either of those two faces,while each of the other bins is accessible through an opening in one ofthe four faces. Mobile drive unit 20 may be configured to rotateinventory holder 30 at appropriate times to present a particular faceand the bins associated with that face to an operator or othercomponents of inventory system 10.

Inventory items represent any objects suitable for storage, retrieval,and/or processing in an automated inventory system 10. For the purposesof this description, “inventory items” may represent any one or moreobjects of a particular type that are stored in inventory system 10.Thus, a particular inventory holder 30 is currently “storing” aparticular inventory item if the inventory holder 30 currently holds oneor more units of that type. As one example, inventory system 10 mayrepresent a mail order warehouse facility, and inventory items mayrepresent merchandise stored in the warehouse facility. Duringoperation, mobile drive units 20 may retrieve inventory holders 30containing one or more inventory items requested in an order to bepacked for delivery to a customer or inventory holders 30 carryingpallets containing aggregated collections of inventory items forshipment. Moreover, in particular embodiments of inventory system 10,boxes containing completed orders may themselves represent inventoryitems.

In particular embodiments, inventory system 10 may also include one ormore inventory stations 50. Inventory stations 50 represent locationsdesignated for the completion of particular tasks involving inventoryitems. Such tasks may include the removal of inventory items frominventory holders 30, the introduction of inventory items into inventoryholders 30, the counting of inventory items in inventory holders 30, thedecomposition of inventory items (e.g. from pallet- or case-sized groupsto individual inventory items), the consolidation of inventory itemsbetween inventory holders 30, and/or the processing or handling ofinventory items in any other suitable manner. In particular embodiments,inventory stations 50 may just represent the physical locations where aparticular task involving inventory items can be completed withinworkspace 70. In alternative embodiments, inventory stations 50 mayrepresent both the physical location and also any appropriate equipmentfor processing or handling inventory items, such as scanners formonitoring the flow of inventory items in and out of inventory system10, communication interfaces for communicating with management module15, and/or any other suitable components. Inventory stations 50 may becontrolled, entirely or in part, by human operators or may be fullyautomated. Moreover, the human or automated operators of inventorystations 50 may be capable of performing certain tasks to inventoryitems, such as packing, counting, or transferring inventory items, aspart of the operation of inventory system 10.

Workspace 70 represents an area associated with inventory system 10 inwhich mobile drive units 20 can move and/or inventory holders 30 can bestored. For example, workspace 70 may represent all or part of the floorof a mail-order warehouse in which inventory system 10 operates.Although FIG. 1 shows, for the purposes of illustration, an embodimentof inventory system 10 in which workspace 70 includes a fixed,predetermined, and finite physical space, particular embodiments ofinventory system 10 may include mobile drive units 20 and inventoryholders 30 that are configured to operate within a workspace 70 that isof variable dimensions and/or an arbitrary geometry. While FIG. 1illustrates a particular embodiment of inventory system 10 in whichworkspace 70 is entirely enclosed in a building, alternative embodimentsmay utilize workspaces 70 in which some or all of the workspace 70 islocated outdoors, within a vehicle (such as a cargo ship), or otherwiseunconstrained by any fixed structure.

In operation, management module 15 selects appropriate components tocomplete particular tasks and transmits task assignments 18 to theselected components to trigger completion of the relevant tasks. Eachtask assignment 18 defines one or more tasks to be completed by aparticular component. These tasks may relate to the retrieval, storage,replenishment, and counting of inventory items and/or the management ofmobile drive units 20, inventory holders 30, inventory stations 50 andother components of inventory system 10. Depending on the component andthe task to be completed, a particular task assignment 18 may identifylocations, components, and/or actions associated with the correspondingtask and/or any other appropriate information to be used by the relevantcomponent in completing the assigned task.

In particular embodiments, management module 15 generates taskassignments 18 based, in part, on inventory requests that managementmodule 15 receives from other components of inventory system 10 and/orfrom external components in communication with management module 15.These inventory requests identify particular operations to be completedinvolving inventory items stored or to be stored within inventory system10 and may represent communication of any suitable form. For example, inparticular embodiments, an inventory request may represent a shippingorder specifying particular inventory items that have been purchased bya customer and that are to be retrieved from inventory system 10 forshipment to the customer. Management module 15 may also generate taskassignments 18 independently of such inventory requests, as part of theoverall management and maintenance of inventory system 10. For example,management module 15 may generate task assignments 18 in response to theoccurrence of a particular event (e.g., in response to a mobile driveunit 20 requesting a space to park), according to a predeterminedschedule (e.g., as part of a daily start-up routine), or at anyappropriate time based on the configuration and characteristics ofinventory system 10. After generating one or more task assignments 18,management module 15 transmits the generated task assignments 18 toappropriate components for completion of the corresponding task. Therelevant components then execute their assigned tasks.

With respect to mobile drive units 20 specifically, management module 15may, in particular embodiments, communicate task assignments 18 toselected mobile drive units 20 that identify one or more destinationsfor the selected mobile drive units 20. Management module 15 may selecta mobile drive unit 20 to assign the relevant task based on the locationor state of the selected mobile drive unit 20, an indication that theselected mobile drive unit 20 has completed a previously-assigned task,a predetermined schedule, and/or any other suitable consideration. Thesedestinations may be associated with an inventory request the managementmodule 15 is executing or a management objective the management module15 is attempting to fulfill. For example, the task assignment may definethe location of an inventory holder 30 to be retrieved, an inventorystation 50 to be visited, a storage location where the mobile drive unit20 should park until receiving another task, or a location associatedwith any other task appropriate based on the configuration,characteristics, and/or state of inventory system 10, as a whole, orindividual components of inventory system 10. For example, in particularembodiments, such decisions may be based on the popularity of particularinventory items, the staffing of a particular inventory station 50, thetasks currently assigned to a particular mobile drive unit 20, and/orany other appropriate considerations.

As part of completing these tasks mobile drive units 20 may dock withand transport inventory holders 30 within workspace 70. Mobile driveunits 20 may dock with inventory holders 30 by connecting to, lifting,and/or otherwise interacting with inventory holders 30 in any othersuitable manner so that, when docked, mobile drive units 20 are coupledto and/or support inventory holders 30 and can move inventory holders 30within workspace 70. While the description below focuses on particularembodiments of mobile drive unit 20 and inventory holder 30 that areconfigured to dock in a particular manner, alternative embodiments ofmobile drive unit 20 and inventory holder 30 may be configured to dockin any manner suitable to allow mobile drive unit 20 to move inventoryholder 30 within workspace 70. Additionally, as noted below, inparticular embodiments, mobile drive units 20 represent all or portionsof inventory holders 30. In such embodiments, mobile drive units 20 maynot dock with inventory holders 30 before transporting inventory holders30 and/or mobile drive units 20 may each remain continually docked witha particular inventory holder 30.

While the appropriate components of inventory system 10 completeassigned tasks, management module 15 may interact with the relevantcomponents to ensure the efficient use of space, equipment, manpower,and other resources available to inventory system 10. As one specificexample of such interaction, management module 15 is responsible, inparticular embodiments, for planning the paths mobile drive units 20take when moving within workspace 70 and for allocating use of aparticular portion of workspace 70 to a particular mobile drive unit 20for purposes of completing an assigned task. In such embodiments, mobiledrive units 20 may, in response to being assigned a task, request a pathto a particular destination associated with the task. Moreover, whilethe description below focuses on one or more embodiments in which mobiledrive unit 20 requests paths from management module 15, mobile driveunit 20 may, in alternative embodiments, generate its own paths.

Components of inventory system 10 may provide information to managementmodule 15 regarding their current state, other components of inventorysystem 10 with which they are interacting, and/or other conditionsrelevant to the operation of inventory system 10. This may allowmanagement module 15 to utilize feedback from the relevant components toupdate algorithm parameters, adjust policies, or otherwise modify itsdecision-making to respond to changes in operating conditions or theoccurrence of particular events.

In addition, while management module 15 may be configured to managevarious aspects of the operation of the components of inventory system10, in particular embodiments, the components themselves may also beresponsible for decision-making relating to certain aspects of theiroperation, thereby reducing the processing load on management module 15.

Thus, based on its knowledge of the location, current state, and/orother characteristics of the various components of inventory system 10and an awareness of all the tasks currently being completed, managementmodule 15 can generate tasks, allot usage of system resources, andotherwise direct the completion of tasks by the individual components ina manner that optimizes operation from a system-wide perspective.Moreover, by relying on a combination of both centralized, system-widemanagement and localized, component-specific decision-making, particularembodiments of inventory system 10 may be able to support a number oftechniques for efficiently executing various aspects of the operation ofinventory system 10. As a result, particular embodiments of managementmodule 15 may, by implementing one or more management techniquesdescribed below, enhance the efficiency of inventory system 10 and/orprovide other operational benefits.

FIG. 2 illustrates in greater detail the components of a particularembodiment of management module 15. As shown, the example embodimentincludes a resource scheduling module 92, a route planning module 94, asegment reservation module 96, an inventory module 97, a communicationinterface module 98, a processor 90, and a memory 91. Management module15 may represent a single component, multiple components located at acentral location within inventory system 10, or multiple componentsdistributed throughout inventory system 10. For example, managementmodule 15 may represent components of one or more mobile drive units 20that are capable of communicating information between the mobile driveunits 20 and coordinating the movement of mobile drive units 20 withinworkspace 70. In general, management module 15 may include anyappropriate combination of hardware and/or software suitable to providethe described functionality.

Processor 90 is operable to execute instructions associated with thefunctionality provided by management module 15. Processor 90 maycomprise one or more general purpose computers, dedicatedmicroprocessors, or other processing devices capable of communicatingelectronic information. Examples of processor 90 include one or moreapplication-specific integrated circuits (ASICs), field programmablegate arrays (FPGAs), digital signal processors (DSPs) and any othersuitable specific or general purpose processors.

Memory 91 stores processor instructions, inventory requests, reservationinformation, state information for the various components of inventorysystem 10 and/or any other appropriate values, parameters, orinformation utilized by management module 15 during operation. Memory 91may represent any collection and arrangement of volatile or nonvolatile,local or remote devices suitable for storing data. Examples of memory 91include, but are not limited to, random access memory (RAM) devices,read only memory (ROM) devices, magnetic storage devices, opticalstorage devices or any other suitable data storage devices.

Resource scheduling module 92 processes received inventory requests andgenerates one or more assigned tasks to be completed by the componentsof inventory system 10. Resource scheduling module 92 may also selectone or more appropriate components for completing the assigned tasksand, using communication interface module 98, communicate the assignedtasks to the relevant components. Additionally, resource schedulingmodule 92 may also be responsible for generating assigned tasksassociated with various management operations, such as prompting mobiledrive units 20 to recharge batteries or have batteries replaced,instructing inactive mobile drive units 20 to park in a location outsidethe anticipated traffic flow or a location near the anticipated site offuture tasks, and/or directing mobile drive units 20 selected for repairor maintenance to move towards a designated maintenance station.

Route planning module 94 receives route requests from mobile drive units20. These route requests identify one or more destinations associatedwith a task the requesting mobile drive unit 20 is executing. Inresponse to receiving a route request, route planning module 94generates a path to one or more destinations identified in the routerequest. Route planning module 94 may implement any appropriatealgorithms utilizing any appropriate parameters, factors, and/orconsiderations to determine the appropriate path. After generating anappropriate path, route planning module 94 transmits a route responseidentifying the generated path to the requesting mobile drive unit 20using communication interface module 98.

Segment reservation module 96 receives reservation requests from mobiledrive units 20 attempting to move along paths generated by routeplanning module 94. These reservation requests request the use of aparticular portion of workspace 70 (referred to herein as a “segment”)to allow the requesting mobile drive unit 20 to avoid collisions withother mobile drive units 20 while moving across the reserved segment. Inresponse to received reservation requests, segment reservation module 96transmits a reservation response granting or denying the reservationrequest to the requesting mobile drive unit 20 using the communicationinterface module 98.

The inventory module 97 maintains information about the location andnumber of inventory items 40 in the inventory system 10. Information canbe maintained about the number of inventory items 40 in a particularinventory holder 30, and the maintained information can include thelocation of those inventory items 40 in the inventory holder 30. Theinventory module 97 can also communicate with the mobile drive units 20,utilizing task assignments 18 to maintain, replenish or move inventoryitems 40 within the inventory system 10.

Communication interface module 98 facilitates communication betweenmanagement module 15 and other components of inventory system 10,including reservation responses, reservation requests, route requests,route responses, and task assignments. These reservation responses,reservation requests, route requests, route responses, and taskassignments may represent communication of any form appropriate based onthe capabilities of management module 15 and may include any suitableinformation. Depending on the configuration of management module 15,communication interface module 98 may be responsible for facilitatingeither or both of wired and wireless communication between managementmodule 15 and the various components of inventory system 10. Inparticular embodiments, management module 15 may communicate usingcommunication protocols such as 802.11, Bluetooth, or Infrared DataAssociation (IrDA) standards. Furthermore, management module 15 may, inparticular embodiments, represent a portion of mobile drive unit 20 orother components of inventory system 10. In such embodiments,communication interface module 98 may facilitate communication betweenmanagement module 15 and other parts of the same system component.

In general, resource scheduling module 92, route planning module 94,segment reservation module 96, inventory module 97, and communicationinterface module 98 may each represent any appropriate hardware and/orsoftware suitable to provide the described functionality. In addition,as noted above, management module 15 may, in particular embodiments,represent multiple different discrete components and any or all ofresource scheduling module 92, route planning module 94, segmentreservation module 96, inventory module 97, and communication interfacemodule 98 may represent components physically separate from theremaining elements of management module 15. Moreover, any two or more ofresource scheduling module 92, route planning module 94, segmentreservation module 96, inventory module 97, and communication interfacemodule 98 may share common components. For example, in particularembodiments, resource scheduling module 92, route planning module 94,segment reservation module 96, and inventory module 97 representcomputer processes executing on processor 90 and communication interfacemodule 98 comprises a wireless transmitter, a wireless receiver, and arelated computer process executing on processor 90.

FIGS. 3 and 4 illustrate in greater detail the components of aparticular embodiment of mobile drive unit 20. In particular, FIGS. 3and 4 include a front and side view of an example mobile drive unit 20.Mobile drive unit 20 includes a docking head 110, a drive module 120, adocking actuator 130, and a control module 170. Additionally, mobiledrive unit 20 may include one or more sensors configured to detect ordetermine the location of mobile drive unit 20, inventory holder 30,and/or other appropriate elements of inventory system 10. In theillustrated embodiment, mobile drive unit 20 includes a position sensor140, a holder sensor 150, an obstacle sensor 160, and an identificationsignal transmitter 162.

Docking head 110, in particular embodiments of mobile drive unit 20,couples mobile drive unit 20 to inventory holder 30 and/or supportsinventory holder 30 when mobile drive unit 20 is docked to inventoryholder 30. Docking head 110 may additionally allow mobile drive unit 20to maneuver inventory holder 30, such as by lifting inventory holder 30,propelling inventory holder 30, rotating inventory holder 30, and/ormoving inventory holder 30 in any other appropriate manner. Docking head110 may also include any appropriate combination of components, such asribs, spikes, and/or corrugations, to facilitate such manipulation ofinventory holder 30. For example, in particular embodiments, dockinghead 110 may include a high-friction portion that abuts a portion ofinventory holder 30 while mobile drive unit 20 is docked to inventoryholder 30. In such embodiments, frictional forces created between thehigh-friction portion of docking head 110 and a surface of inventoryholder 30 may induce translational and rotational movement in inventoryholder 30 when docking head 110 moves and rotates, respectively. As aresult, mobile drive unit 20 may be able to manipulate inventory holder30 by moving or rotating docking head 110, either independently or as apart of the movement of mobile drive unit 20 as a whole.

Drive module 120 propels mobile drive unit 20 and, when mobile driveunit 20 and inventory holder 30 are docked, inventory holder 30. Drivemodule 120 may represent any appropriate collection of componentsoperable to propel drive module 120. For example, in the illustratedembodiment, drive module 120 includes motorized axle 122, a pair ofmotorized wheels 124, and a pair of stabilizing wheels 126. Onemotorized wheel 124 is located at each end of motorized axle 122, andone stabilizing wheel 126 is positioned at each end of mobile drive unit20.

Docking actuator 130 moves docking head 110 towards inventory holder 30to facilitate docking of mobile drive unit 20 and inventory holder 30.Docking actuator 130 may also be capable of adjusting the position ororientation of docking head 110 in other suitable manners to facilitatedocking. Docking actuator 130 may include any appropriate components,based on the configuration of mobile drive unit 20 and inventory holder30, for moving docking head 110 or otherwise adjusting the position ororientation of docking head 110. For example, in the illustratedembodiment, docking actuator 130 includes a motorized shaft (not shown)attached to the center of docking head 110. The motorized shaft isoperable to lift docking head 110 as appropriate for docking withinventory holder 30.

Drive module 120 may be configured to propel mobile drive unit 20 in anyappropriate manner. For example, in the illustrated embodiment,motorized wheels 124 are operable to rotate in a first direction topropel mobile drive unit 20 in a forward direction. Motorized wheels 124are also operable to rotate in a second direction to propel mobile driveunit 20 in a backward direction. In the illustrated embodiment, drivemodule 120 is also configured to rotate mobile drive unit 20 by rotatingmotorized wheels 124 in different directions from one another or byrotating motorized wheels 124 at different speed from one another.

Position sensor 140 represents one or more sensors, detectors, or othercomponents suitable for determining the location of mobile drive unit 20in any appropriate manner. For example, in particular embodiments, theworkspace 70 associated with inventory system 10 includes a number offiducial marks that mark points on a two-dimensional grid that coversall or a portion of workspace 70. In such embodiments, position sensor140 may include a camera and suitable image- and/or video-processingcomponents, such as an appropriately-programmed digital signalprocessor, to allow position sensor 140 to detect fiducial marks withinthe camera's field of view. Control module 170 may store locationinformation that position sensor 140 updates as position sensor 140detects fiducial marks. As a result, position sensor 140 may utilizefiducial marks to maintain an accurate indication of the location mobiledrive unit 20 and to aid in navigation when moving within workspace 70.

Holder sensor 150 represents one or more sensors, detectors, or othercomponents suitable for detecting inventory holder 30 and/ordetermining, in any appropriate manner, the location of inventory holder30, as an absolute location or as a position relative to mobile driveunit 20. Holder sensor 150 may be capable of detecting the location of aparticular portion of inventory holder 30 or inventory holder 30 as awhole. Mobile drive unit 20 may then use the detected information fordocking with or otherwise interacting with inventory holder 30.

Obstacle sensor 160 represents one or more sensors capable of detectingobjects located in one or more different directions in which mobiledrive unit 20 is capable of moving. Obstacle sensor 160 may utilize anyappropriate components and techniques, including optical, radar, sonar,pressure-sensing and/or other types of detection devices appropriate todetect objects located in the direction of travel of mobile drive unit20. In particular embodiments, obstacle sensor 160 may transmitinformation describing objects it detects to control module 170 to beused by control module 170 to identify obstacles and to take appropriateremedial actions to prevent mobile drive unit 20 from colliding withobstacles and/or other objects.

Obstacle sensor 160 may also detect signals transmitted by other mobiledrive units 20 operating in the vicinity of the illustrated mobile driveunit 20. For example, in particular embodiments of inventory system 10,one or more mobile drive units 20 may include an identification signaltransmitter 162 that transmits a drive identification signal. The driveidentification signal indicates to other mobile drive units 20 that theobject transmitting the drive identification signal is in fact a mobiledrive unit. Identification signal transmitter 162 may be capable oftransmitting infrared, ultraviolet, audio, visible light, radio, and/orother suitable signals that indicate to recipients that the transmittingdevice is a mobile drive unit 20.

Additionally, in particular embodiments, obstacle sensor 160 may also becapable of detecting state information transmitted by other mobile driveunits 20. For example, in particular embodiments, identification signaltransmitter 162 may be capable of including state information relatingto mobile drive unit 20 in the transmitted identification signal. Thisstate information may include, but is not limited to, the position,velocity, direction, and the braking capabilities of the transmittingmobile drive unit 20. In particular embodiments, mobile drive unit 20may use the state information transmitted by other mobile drive units toavoid collisions when operating in close proximity with those othermobile drive units.

Control module 170 monitors and/or controls operation of drive module120 and docking actuator 130. Control module 170 may also receiveinformation from sensors such as position sensor 140 and holder sensor150 and adjust the operation of drive module 120, docking actuator 130,and/or other components of mobile drive unit 20 based on thisinformation. Additionally, in particular embodiments, mobile drive unit20 may be configured to communicate with a management device ofinventory system 10 and control module 170 may receive commandstransmitted to mobile drive unit 20 and communicate information back tothe management device utilizing appropriate communication components ofmobile drive unit 20. Control module 170 may include any appropriatehardware and/or software suitable to provide the describedfunctionality. In particular embodiments, control module 170 includes ageneral-purpose microprocessor programmed to provide the describedfunctionality. Additionally, control module 170 may include all orportions of docking actuator 130, drive module 120, position sensor 140,and/or holder sensor 150, and/or share components with any of theseelements of mobile drive unit 20.

Moreover, in particular embodiments, control module 170 may includehardware and software located in components that are physically distinctfrom the device that houses drive module 120, docking actuator 130,and/or the other components of mobile drive unit 20 described above. Forexample, in particular embodiments, each mobile drive unit 20 operatingin inventory system 10 may be associated with a software process(referred to here as a “drive agent”) operating on a server that is incommunication with the device that houses drive module 120, dockingactuator 130, and other appropriate components of mobile drive unit 20.This drive agent may be responsible for requesting and receiving tasks,requesting and receiving routes, transmitting state informationassociated with mobile drive unit 20, and/or otherwise interacting withmanagement module 15 and other components of inventory system 10 onbehalf of the device that physically houses drive module 120, dockingactuator 130, and the other appropriate components of mobile drive unit20. As a result, for the purposes of this description and the claimsthat follow, the term “mobile drive unit” includes software and/orhardware, such as agent processes, that provides the describedfunctionality on behalf of mobile drive unit 20 but that may be locatedin physically distinct devices from the drive module 120, dockingactuator 130, and/or the other components of mobile drive unit 20described above.

While FIGS. 3 and 4 illustrate a particular embodiment of mobile driveunit 20 containing certain components and configured to operate in aparticular manner, mobile drive unit 20 may represent any appropriatecomponent and/or collection of components configured to transport and/orfacilitate the transport of inventory holders 30. As another example,mobile drive unit 20 may represent part of an overhead crane system inwhich one or more crane assemblies are capable of moving within anetwork of wires or rails to a position suitable to dock with aparticular inventory holder 30. After docking with inventory holder 30,the crane assembly may then lift inventory holder 30 and move inventoryto another location for purposes of completing an assigned task.

Furthermore, in particular embodiments, mobile drive unit 20 mayrepresent all or a portion of inventory holder 30. Inventory holder 30may include motorized wheels or any other components suitable to allowinventory holder 30 to propel itself. As one specific example, a portionof inventory holder 30 may be responsive to magnetic fields. Inventorysystem 10 may be able to generate one or more controlled magnetic fieldscapable of propelling, maneuvering and/or otherwise controlling theposition of inventory holder 30 as a result of the responsive portion ofinventory holder 30. In such embodiments, mobile drive unit 20 mayrepresent the responsive portion of inventory holder 30 and/or thecomponents of inventory system 10 responsible for generating andcontrolling these magnetic fields. While this description providesseveral specific examples, mobile drive unit 20 may, in general,represent any appropriate component and/or collection of componentsconfigured to transport and/or facilitate the transport of inventoryholders 30.

FIG. 5 illustrates in greater detail the components of a particularembodiment of inventory holder 30. In particular, FIG. 5 illustrates thestructure and contents of one side of an example inventory holder 30. Ina particular embodiment, inventory holder 30 may comprise any number offaces with similar or different structure. As illustrated, inventoryholder 30 includes a frame 310, a plurality of legs 328, and a dockingsurface 350.

Frame 310 holds inventory items 40. Frame 310 provides storage space forstoring inventory items 40 external or internal to frame 310. Thestorage space provided by frame 310 may be divided into a plurality ofinventory bins 320, each capable of holding inventory items 40.Inventory bins 320 may include any appropriate storage elements, such asbins, compartments, or hooks.

In a particular embodiment, frame 310 is composed of a plurality oftrays 322 stacked upon one another and attached to or stacked on a base318. In such an embodiment, inventory bins 320 may be formed by aplurality of adjustable dividers 324 that may be moved to resize one ormore inventory bins 320. In alternative embodiments, frame 310 mayrepresent a single inventory bin 320 that includes a single tray 322 andno adjustable dividers 324. Additionally, in particular embodiments,frame 310 may represent a load-bearing surface mounted on mobilityelement 330. Inventory items 40 may be stored on such an inventoryholder 30 by being placed on frame 310. In general, frame 310 mayinclude storage internal and/or external storage space divided into anyappropriate number of inventory bins 320 in any appropriate manner.

Additionally, in a particular embodiment, frame 310 may include aplurality of device openings 326 that allow mobile drive unit 20 toposition docking head 110 adjacent docking surface 350. The size, shape,and placement of device openings 326 may be determined based on thesize, the shape, and other characteristics of the particular embodimentof mobile drive unit 20 and/or inventory holder 30 utilized by inventorysystem 10. For example, in the illustrated embodiment, frame 310includes four legs 328 that form device openings 326 and allow mobiledrive unit 20 to position mobile drive unit 20 under frame 310 andadjacent to docking surface 350. The length of legs 328 may bedetermined based on a height of mobile drive unit 20.

Docking surface 350 comprises a portion of inventory holder 30 thatcouples to, abuts, and/or rests upon a portion of docking head 110, whenmobile drive unit 20 is docked to inventory holder 30. Additionally,docking surface 350 supports a portion or all of the weight of inventoryholder 30 while inventory holder 30 is docked with mobile drive unit 20.The composition, shape, and/or texture of docking surface 350 may bedesigned to facilitate maneuvering of inventory holder 30 by mobiledrive unit 20. For example, as noted above, in particular embodiments,docking surface 350 may comprise a high-friction portion. When mobiledrive unit 20 and inventory holder 30 are docked, frictional forcesinduced between docking head 110 and this high-friction portion mayallow mobile drive unit 20 to maneuver inventory holder 30.Additionally, in particular embodiments, docking surface 350 may includeappropriate components suitable to receive a portion of docking head110, couple inventory holder 30 to mobile drive unit 20, and/orfacilitate control of inventory holder 30 by mobile drive unit 20.

Holder identifier 360 marks a predetermined portion of inventory holder30 and mobile drive unit 20 may use holder identifier 360 to align withinventory holder 30 during docking and/or to determine the location ofinventory holder 30. More specifically, in particular embodiments,mobile drive unit 20 may be equipped with components, such as holdersensor 150, that can detect holder identifier 360 and determine itslocation relative to mobile drive unit 20. As a result, mobile driveunit 20 may be able to determine the location of inventory holder 30 asa whole. For example, in particular embodiments, holder identifier 360may represent a reflective marker that is positioned at a predeterminedlocation on inventory holder 30 and that holder sensor 150 can opticallydetect using an appropriately-configured camera.

Depending on the configuration and characteristics of mobile drive unit20 and inventory system 10, mobile drive unit 20 may move inventoryholder 30 using a variety of appropriate methods. In a particularembodiment, mobile drive unit 20 is capable of moving inventory holder30 along a two-dimensional grid, combining movement along straight-linesegments with ninety-degree rotations and arcing paths to transportinventory holder 30 from the first location to the second location.Additionally, while moving, mobile drive unit 20 may use fixed objectslocated in the workspace as reference points to assist in navigation.For example, in particular embodiments, inventory system 10 includesmultiple fiducial marks. Mobile drive unit 20 may be configured todetect fiducial marks and to determine the location of mobile drive unit20 and/or measure its movement based on the detection of fiducial marks.

After mobile drive unit 20 arrives at the second location, mobile driveunit 20 may perform appropriate operations to facilitate access toinventory items 40 stored in inventory holder 30. For example, mobiledrive unit 20 may rotate inventory holder 30 to present a particularface of inventory holder 30 to an operator of inventory system 10 orother suitable party, such as a packer selecting inventory items 40 frominventory holder 30. Mobile drive unit 20 may also undock from inventoryholder 30. Alternatively, instead of undocking at the second location,mobile drive unit 20 may transport inventory holder 30 back to the firstlocation or to a third location after any appropriate actions have beentaken involving inventory items 40. For example, after a packer hasremoved particular inventory items 40 from inventory holder 30, mobiledrive unit 20 may return inventory holder 30 to its original storagelocation, a new storage location, or another inventory station. Mobiledrive unit 20 may then undock from inventory holder 30 at this newlocation.

As described above, embodiments herein are directed to management ofreusable containers that are used for transporting inventory items in aninventory system, as well as techniques related to selection,organization, and management of components associated with operationsinvolving the containers.

FIG. 6 illustrates a particular embodiment of reusable containers 402(individually, a container 402) and an associated container holder 406.Each container 402 may be a container used to transport inventory items40 independently of inventory holders 30. Containers 402 may all be thesame structure, or may have different structures. For ease ofdescription, however, some embodiments herein describe a singlecontainer 402, but the system is typically implemented with multiplecontainers, which may be the same or different in size and/or designfrom one another. In an example, the container 402 can be a container inwhich inventory items 40 are initially delivered to an inventory system10. In a particular embodiment, the container 402 is a box constructedof durable material (such as plastic) and includes one or more flaps 404that can be rotated upon hinges to close a top of the box. In someembodiments, at least some containers 402 include flexible material(such as cloth), lack tops, or otherwise differ in size, shape, or othercharacteristics from the container 402 depicted in FIG. 6. In aparticular embodiment, the containers 402 are constructed in astandardized size or set of sizes, which can accommodate individual orgroups of inventory items 40 of differing sizes.

The container holder 406 carries containers 402 and may be sized tocarry multiple containers 402 and/or containers 402 of different sizes.For example, the container holder 406 may include one or more berths,e.g., shelves, slots, or other storage spaces, each sized to carry oneor more containers 402. The container holder 406 permits containers 402to be easily loaded or removed from the container holder 406 by human orautomated operators. In a particular embodiment, the container holder406 may be an inventory holder 30 configured to carry the container 402.For example, the container holder 406 may have various features similarto the inventory holder 30 as described above with respect to FIG. 5,such as legs 428 and a docking surface 450 by which mobile drive unitscan dock with the container holder 406.

FIG. 7 illustrates an inventory system 510 that includes containerholders 506 according to a particular embodiment. The inventory system510 includes management module 515, one or more mobile drive units 520,one or more inventory holders 530, and one or more inventory stations550. All of these components can provide the same functionality aspreviously described with respect to other components of the same nameand similar reference numbers and furthermore may provide additionalfunctionality related to the container holders 506. For example, whereasthe mobile drive units 520 may transport inventory holders 530 betweenpoints within a workspace 70 in response to commands communicated bymanagement module 15, the mobile drive units 520 may likewise transportcontainer holders 506. As another example, tasks designated forcompletion at inventory stations 550 may involve either or both ofinventory holders 530 or container holders 506.

The inventory system 510 includes container holders 506 in a variety ofstates. For example, an “empty” subset of container holders 506 (denotedby the letter “E” in FIG. 7) can carry empty containers 402 that do notcurrently contain any inventory items 40. An “inbound” subset ofcontainer holders 506 (denoted by the letter “I” in FIG. 7) can carrycontainers 402 with inbound inventory items 40, e.g., inventory itemsreceived into the inventory system 510 from merchants or vendors, whilean “outbound” subset of container holders 506 (denoted by the letter “O”in FIG. 7) can carry containers 402 with outbound inventory items 40,e.g., inventory items to be sent outside the inventory system 510 tofulfill customer orders or to other facilities. An “unoccupied” subsetof container holders 506 (denoted by the letter “U” in FIG. 7) can be atleast partially vacant and unoccupied by containers 402. States mayapply to an entire container holder 506 or may refer to a condition ofan individual berth of a container holder 506. As such, a particularcontainer holder 506 may simultaneously be in multiple states or subsetsas a result of different berths of the container holder 506 being indiffering states.

As described in greater detail below with respect to FIG. 8, tasks maybe performed at a number of types of stations 550 to cause containerholders 506 (or particular berths thereof) to change state. Types ofstations 550 shown in FIG. 7 include induct stations 550 a, stowstations 550 b, pick stations 550 c, discharge stations 550 d, andadjustment stations 550 e. In some embodiments, a particular station 550can function as multiple types of stations at once and/or be switchedbetween types. In some embodiments, the management module 515 can assigna particular station 550 to change from one type to another based onfluctuating need for different tasks to be performed. The workspace 570can also include one or more storage spaces 508 in which containerholders 506 can be stored, such as when a station 550 is not immediatelyavailable for a next task associated with the container holder 506. Insome embodiments, particular storage spaces 508 may be specificallydesignated for container holders 506 or container holders 506 in aparticular state. Particular storage spaces 508 in some embodiments arepart of a mixed storage area equally available to container holders 506,regardless of state, and/or inventory holders 530.

FIG. 8 is a flow chart illustrating management of container holders 606in use in an inventory system 610 according to a particular embodiment.Management of the container holders 606 is facilitated by the managementmodule 515. To this end, the management module 515 provides instructionsto various components of the inventory system 510, such as instructingthe movements of the mobile drive units 620 described below andproviding instructions for automated or human operators at stations 550.For example, the management module 515 may provide instructions to humanoperators via audible instructions, visible instructions on a display,light shined on a feature to be interacted with during an instructedtask, or some other indicator. At 652, a container holder 606 in aU-state (i.e., in which at least one berth is unoccupied by, andcarrying zero, containers 602) can be moved to an induct station, suchas the induct station 550 a shown in FIG. 7. For example, the containerholder 606 can be moved by a mobile drive unit 620 that is the sameas—or different from—any mobile drive unit 620 used for any subsequentmoves of the container holder 606. At 654, a human or automated operatorat the induct station, in response to instructions from the managementmodule 515, can load the container holder 606 with containers 602containing inbound inventory items. For example, containers 602delivered to the induct station via conveyers, carts, or other containerholders 606 from other inventory systems may contain items forreplenishing a stock of inventory items 640 stored in the inventorysystem 610. Loading the container holder 606 with containers 602containing inbound inventory items 640 can change, set, and/or convertthe container holder 606 (or a particular berth thereof) from a U-stateto an I-state (i.e., in which at least one berth is carrying one or morecontainers 602 containing inbound inventory items), thus ensuring thatthe container holder 606 will be included in the “inbound” subset. Forexample, as shown in FIG. 7, U-state container holders arriving to theinduct station 550 a can depart as I-state container holders.

At 656 in FIG. 8, the I-state container holder 606 can be moved awayfrom the induct station, such as by a mobile drive unit 620. At 658, theI-state container holder 606 can be moved to and/or from storage 608,for example, if a station is not immediately available for the I-statecontainer holder 606. As will be appreciated from the drawings andsubsequent description, in some embodiments, the storage area 608 may beutilized for container holders 606 in any state, including containerholders 606 with berths in different states (such as the containerholder 606 that is depicted in FIG. 8 at the extreme right of thestorage area 608).

At 660, the I-state container holder 606 can be moved, from either theinduct station or storage 608, to an available stow station, such as thestow station 550 b shown in FIG. 7. At 662, a human or automatedoperator at the stow station can transfer the inbound inventory items640 from the containers 602 carried by the container holder 606 to afirst set of inventory holders 630 moved to the stow station. Theinventory items 640 inbound to the inventory holders 630 may thusreplenish a stock stored in the inventory system 610. Transferring theinbound inventory items 640 from the containers 602 to the inventoryholders 630 can empty the containers 602. Emptying the containers 602can change, set, and/or convert the container holder 606 (or aparticular berth thereof) from an I-state to an E-state (i.e., in whichat least one berth is carrying one or more empty containers 602), thusensuring that the container holder 606 will be included in the “empty”subset. For example, as shown in FIG. 7, I-state container holdersarriving to the stow station 550 b can depart as E-state containerholders.

At 664 in FIG. 8, the E-state container holder 606 can be moved awayfrom the stow station, such as by a mobile drive unit 620. At 666, theE-state container holder 606 can be moved to and/or from storage 608,for example, if a station is not immediately available for the E-statecontainer holder 606.

At 668, the E-state container holder 606 can be moved, from either thestow station or storage 608, to an available pick station, such as thepick station 550 c shown in FIG. 7. At 670, a human or automatedoperator at the pick station can transfer outbound inventory items 640into the empty containers 602 carried by the container holder 606. Theoutbound inventory items 640 can be transferred from a second set ofinventory holders 630 moved to the pick station. A set of inventoryitems 640 outbound from the inventory holders 630 may thus be acquiredand grouped for subsequent tasks facilitated by the inventory system610. Transferring the outbound inventory items 640 from the inventoryholders 630 to the containers 602 can change, set, and/or convert thecontainer holder 606 (or a particular berth thereof) from an E-state toan O-state (i.e., in which at least one berth is carrying one or morecontainers 602 containing outbound inventory items), thus ensuring thatthe container holder 606 will be included in the “outbound” subset. Forexample, as shown in FIG. 7, E-state container holders arriving to thepick station 550 c can depart as O-state container holders.

At 672 in FIG. 8, the O-state container holder 606 can be moved awayfrom the pick station, such as by a mobile drive unit 620. At 674, theO-state container holder 606 can be moved to and/or from storage 608,for example, if a station is not immediately available for the O-statecontainer holder 606.

At 676, the O-state container holder 606 can be moved, from either thepick station or storage 608, to an available discharge station, such asthe discharge station 550 d shown in FIG. 7. At 678, a human orautomated operator at the discharge station can unload outboundinventory items 640 from the containers 602 (and/or entire containers602 containing outbound inventory items 640) from the container holder606. For example, the containers 602 of outbound inventory items can betransferred to conveyers, carts, or other container holders 606 fromother inventory systems to discharge the containers 602 and associatedcontents from the inventory system 610. Unloading the containers 602 ofoutbound inventory items 640 from the container holder 606 can change,set, and/or convert the container holder 606 (or a particular berththereof) from an O-state to the U-state (i.e., back to the state inwhich at least one berth is unoccupied by containers 602), thus ensuringthat the container holder 606 will be included in the “unoccupied”subset. For example, as shown in FIG. 7, O-state container holdersarriving to the discharge station 550 d can depart as U-state containerholders.

At 680 in FIG. 8, the U-state container holder 606 can be moved awayfrom the discharge station, such as by a mobile drive unit 620. At 682,the U-state container holder 606 can be moved to and/or from storage608, for example, if a station is not immediately available for theU-state container holder 606. The U-state container holder 606, aspreviously described, at 652, can be moved, from either the dischargestation or storage 608, to an induct station, such as the induct station550 a shown in FIG. 7.

In some embodiments, introducing or removing extra empty containers 602may be desirable. As an illustrative example, inbound containers 602 maycontain a lower average number of inventory items 40 than outboundcontainers 602 such that emptied containers 602 are generated fasterthan consumed. On the other hand, if inbound containers 602 contain ahigher average number of inventory items 40 than outbound containers602, the demand for emptied containers 602 may exceed the supply. As afurther illustrative example, a delay in transferring inbound inventoryitems 640 out of containers 602 at a pick station (as at 662) may resultin a shortage of empty containers 602 for filling with outboundinventory items 640 at a stow station (as at 670). On the other hand, adelay in unloading containers 602 at a discharge station (as at 678) mayresult in a shortage of U-state container holders 606 for loadingcontainers 602 of inbound inventory items 640 at an induct station (asat 654). To this end, in some embodiments, empty containers 602 can beadded to and/or removed from a container holder 606 (as at 690) at anadjusting station to switch the container holder 606 (or a particularberth thereof) between the U-state and the E-state. In some embodiments,a U-state container holder 606 can be moved (as at 684) to an adjustingstation, loaded with empty containers 602 (as at 690), and transportedas an E-state container holder 606 to storage (as at 686 and 666) or toa pick station (as at 686 and 668). Similarly, an E-state containerholder 606 can be moved (as at 686) to an adjusting station, unloaded ofempty containers 602 (as at 690), and transported as a U-state containerholder 606 to storage (as at 684 and 682) or to an induct station (as at684 and 652). As may be appreciated with respect to FIG. 7, U-statecontainer holders arriving to the adjustment station 550 e can depart asE-state container holders, and E-state container holders 606 arriving tothe adjustment station 550 e can depart as U-state container holders606. In some embodiments, empty containers 602 can be directly added toa U-state container holder 606 at another station 550, such as an inductstation 550 a or any other station. Empty containers 602 in someembodiments can be removed from an E-state container holder 606 atanother station 550, such as a discharge station 550 d or any otherstation.

In some embodiments, moving inventory items 640 directly betweencontainers 602 may be desirable. For example, at the stow station at662, rather than transferring inbound inventory items 640 to a first setof inventory holders 630, the inbound inventory items 640 may betransferred to other containers 602 carried by the same or a differentcontainer holder 606. As another example, at the pick station at 670,rather than transferring outbound inventory items 640 to a second set ofinventory holders 630, the outbound inventory items 640 may betransferred to other containers 602 carried by the same or a differentcontainer holder 606. Transferring inventory items between containers602 may permit inventory items 40 to be consolidated organized, ordistributed as desired.

FIG. 9 illustrates in greater detail some components that may beutilized in particular embodiments of the resource scheduling module 92shown in FIG. 2. For example, the resource scheduling module 92 mayinclude an induct container holder selection module 732, a stowcontainer holder selection module 734, a stow inventory holder selectionmodule 746, a pick container holder selection module 736, a pickinventory holder selection module 748, and a discharge container holderselection module 738. The induct container holder selection module 732can select the particular container holder 606 to be moved to an inductstation to facilitate an inducting operation such as 654. The stowcontainer holder selection module 734 can select a particular containerholder 606 to be moved to a stow station to facilitate a stowingoperation such as 662, while the stow inventory holder selection module746 can select a particular inventory holder 630 to be moved to the stowstation to facilitate the stowing operation. The pick container holderselection module 736 can select a particular container holder 606 to bemoved to a pick station to facilitate a pick operation such as 670,while the pick inventory holder selection module 748 can select aparticular inventory holder 630 to be moved to the pick station tofacilitate the pick operation. The discharge container holder selectionmodule 738 can select a particular container holder 606 to be moved to adischarge station to facilitate a discharge operation such as 678.

Although particular examples of selection criteria may be describedherein within the context of a use by a particular selection moduledescribed with respect to FIG. 9, any selection criteria describedherein may alternatively or additionally be utilized by any of suchselection modules. In some embodiments, for an induct operation (such asat 654 shown in FIG. 8), the induct container holder selection module732 selects a U-style container holder 606 based on proximity. Forexample, the U-style container holder 606 nearest to an induct stationmay be selected. However, the induct container holder selection module732 may additionally or alternatively utilize other criteria in theselection process, such as the utility of the storage space 608 occupiedby the U-state container holder 606 for other container holders 606and/or inventory holders 630.

In some embodiments, for a stow operation (such as at 662 shown in FIG.8) the stow container holder selection module 734 selects a containerholder 606 based on a duration factor. For example, the stow containerholder selection module 734 may select the I-state container holder 606that is carrying the oldest inbound containers 602 (i.e., the containers602 that were inducted longest ago).

In some embodiments, for the same stow operation (e.g., at 662 shown inFIG. 8), the stow inventory holder selection module 746 selects aninventory holder 630 to move to the stow station based on distanceand/or fitness of the inventory holder 630 for the inbound inventoryitems 640. For example, the stow inventory holder selection module 746may evaluate whether a particular inventory holder 630 contains adequatespace for storing inbound inventory items 640 contained in the inboundcontainers 602 carried by the container holder 606 selected by the stowcontainer holder selection module 734. The stow inventory holderselection module 746 may select the nearest such suitable inventoryholder 630.

In some embodiments, for a pick operation (such as at 670 shown in FIG.8), the pick container holder selection module 736 selects the containerholder 606 based on proximity. For example, the E-style container holder606 nearest to a pick station may be selected.

In some embodiments, for the same pick operation (e.g., at 670 shown inFIG. 8) the pick inventory holder selection module 748 selects aparticular inventory holder 630 to be moved to the pick station. Thisselection may be based on requests or orders for outbound inventoryitems 640 that are being fulfilled at the pick station. The pickinventory holder selection module 748, in some embodiments, selects aparticular inventory holder 630 carrying a requested inventory item 640based on proximity of the particular inventory holder 630 to the pickstation. In some embodiments, the pick inventory holder selection module748 selects a particular inventory holder 630 based on the anticipateddestination of outbound containers 602 on a container holder 606 at thepick station. Illustrative examples of such destination-based selectionsare illustrated in FIGS. 10-11, and example selections by the dischargecontainer holder selection module 738 are described below, such as withrespect to FIG. 12.

FIG. 10 illustrates an example scenario of determining an allocation ofinventory holders 830 a-f to a pick station 850. A pick station 850 maybe associated with a series of requests, in which each requestcorresponds to a particular inventory item to be transported from astorage location in a particular inventory holder 830 to one of aplurality of other destinations via a container 802. For example, somerequests may include retrieving particular inventory items to betransported to an area where customer orders are boxed for shipping,other requests may involve items to be transported to a location forgift-wrapping, and yet other requests may involve items to betransported to another part of a warehouse, another warehouse, orelsewhere. Completing a request may involve selecting and moving aparticular inventory holder 830 containing the requested item to thepick station 850 so that the item can be loaded into a container 802.The loaded container 802 may be subsequently routed to the designateddestination, for example, by shifting the container 802 from a containerholder 806 (such as at 678 in FIG. 8) and onto a conveyer, cart,different container holder, or other mechanism to convey the loadedcontainer 802 to the designated destination or by directly sending acontainer holder 806 bearing the loaded container 802 (e.g., an O-statecontainer 606 in FIG. 8) to the designated destination. Discussionherein will refer to four such destinations (designated “A”, “S”, “T”,and “G” in FIGS. 10-11); however, embodiments may include any number ofdestinations. In the example scenario depicted in FIG. 10, eachinventory holder 830 a-f is designated “A”, “S”, “T”, or “G”, accordingto the designated destination of a requested inventory item storedtherein.

Inventory items designated for a shared destination, regardless ofwhether obtained from different inventory holders 830, may betransported to the shared destination in a common container 802. Forexample, inventory items designated for the “A” destination (e.g., frominventory holders 830 a, 830 c, and/or 830 e) can be loaded into thecommon container 802 a for transport to the “A” destination. Before anyinventory items are placed in a container 802, the container 802 may bean ‘uncommitted’ container 802 (e.g., containers 802 b, 802 c, or 802 d,designated by a “?” in FIG. 10), equally available to transportinventory items to any of the destinations. However, upon receiving afirst inventory item, the container 802 may become designated to thesame destination as the received inventory item, thus committing thecontainer 802 to receive only other inventory items designated to thesame destination (such as container 802 a designated for destination Abased on, for example, receiving an inventory item from inventory holder830 c). Accordingly, a sequence in which inventory holders 830 are movedmay be organized to favor inventory holders 830 associated with requestsfor inventory items that are designated for a destination of a committedcontainer 802.

In a non-limiting example illustrated in FIG. 10, all four containers802 a, 802 b, 802 c, and 802 d of a primary container holder 806 alocated at a pick station 850 are initially uncommitted. All fourcontainers 802 e, 802 f, 802 g, and 802 h of a secondary containerholder 806 b also located at the pick station 850 are also initiallyuncommitted. Each inventory holder 830 a-f available to fulfill arequest at the pick station 850 is assigned an initial priority score.The priority score (graphically represented in FIG. 10 as a left-rightdistance from the pick station 850, with left-most positionscorresponding to the highest priority scores) is used to determine whichinventory holder 830 a-f will be the next to be moved to the pickstation 850. As such, an inventory holder 830 c having the highestpriority score is moved to the pick station 850. An inventory itemdesignated for destination “A” is transferred from the inventory holder830 c to a container 802 a, thereby converting the container 802 a froman uncommitted state to a state committed to destination “A”.

Continuing in this non-limiting example, the priority scores of theremaining inventory holders 830 a, 830 b, and 830 d-f are adjusted tofavor filling the committed container 802 a over causing an uncommittedcontainer 802 b, 802 c or 802 d to become committed. To this end, thepriority scores of the inventory holders 830 a and 830 e havinginventory items also designated for destination “A” are upgradedrelative to the priority scores of the inventory holders 830 b, 830 d,and 830 f having inventory items not designated for destination “A”. Forexample, the priority scores of the dissimilar inventory holders 830 b,830 d, and 830 f may be reduced (graphically represented in FIG. 10 as arightward shift to the respective score positions 830 b′, 830 d′, and830 f) and/or the priority scores of the remaining similar inventoryholders 830 a and 830 e can be increased (graphically represented inFIG. 10 as a leftward shift to the respective score positions 830 a′ and830 e′). An inventory holder 830 e with another inventory itemdesignated for destination “A” (i.e., the same destination as thecommitted container 802 a) is thus the next to be moved to the pickstation 850 due to having the highest priority score (i.e., as a resultof the upgrade to the increased score position 830 e′ of the inventoryholder 830 e and/or the reduced score positions 830 b′ or 830 f′ ofother inventory holders 830 b or 830 f).

In some embodiments, priority scores may be adjusted every time aninventory item from an inventory holder 830 is transferred into acontainer 802. For example, priority scores may be adjusted after eachpick operation until a committed container 802 is filled, and a newuncommitted container 802 may become committed based on a highestscoring inventory holder 830 at that point.

In some embodiments, adjusting priority scores may not always result ina sequence of only inventory holders 830 matching a committed container802. This may be appreciated by continuing with the non-limiting exampledescribed above with respect to FIG. 10. As described above, aninventory holder 830 c (with an “A” destination item) is moved to thepick station 850 first, causing the container 802 to be committed (alsoto the “A” destination), and the initial adjustment of priority scoresresults in another inventory holder 830 e (also with an “A” destinationitem) to be moved to the pick station 850 second. Yet, without furtherpriority score adjustment, the adjusted priority score position of 830a′ will still be less than either adjusted priority score position of830 b′ or 830 f′. Thus, the inventory holders 830 b and 830 f(respectively with “S” and “T” destination items) will be moved beforemovement of the inventory holder 830 a (with an additional “A”destination item). As a result, in the non-limiting example illustratedin FIG. 10, the container 802 b will be committed to the “S” destination(by inventory items from inventory holder 830 b) and container 802 cwill be committed to the “T” destination (by inventory items frominventory holder 830 f) before the container 802 a committed to the “A”destination receives another inventory item from the inventory holder830 a with the upgraded priority score.

As may thus be appreciated, altering an amount by which priority scoresare adjusted may have a corresponding effect on a probability that aninventory holder 830 matching a committed container 802 will proceednext in sequence. An appropriate amount by which to alter priorityscores may depend on many factors, including but not limited to, a sizeor complexity of the inventory system and the combination of otherparameters considered in determining the priority score. In someaspects, parameters considered in determining the priority score mayinclude a proximity of an ordered item to a pick station 850 or anurgency with which an ordered item is requested.

FIG. 11 illustrates a further example scenario in determining anallocation of inventory holders 830 g and 830 h to a pick station 850.In some embodiments, priority scores of inventory holders 830 may beadjusted based on whether committed containers 802 are already fulland/or whether moving an inventory holder 830 would involve committing acontainer 802 of an additional container holder 806. The scenariodepicted in FIG. 11 is a continuation of the non-limiting exampledescribed above with respect to FIG. 10. Each of the inventory holders830 a-f depicted in FIG. 10 has been ultimately moved to and processedat the pick station 850, causing all of the containers 802 a-d of theprimary container holder 806 a to be committed. Additional inventoryholders 830 were moved to and processed at the pick station 850 untilcontainers 802 a (“A” destination), 802 b (“S” destination), and 802 c(“T” destination) are each full (denoted by a marked upper left cornerof each container 802 a-c in FIG. 11). Container 802 d has space toreceive one more “G” destination item, and an available inventory holder830 g (with an “A” destination item) has a priority score significantlyhigher than another available inventory holder 830 h (with a “G”destination item”). Based on a determination that the primary containerholder 806 a lacks acceptable space for an “A” destination item (e.g.,that the “A” destination container 802 a is full and all othercontainers 802 b-d are otherwise committed), the priority scores of allinventory holders 830 with “A” destination items can be downgradedrelative to the priority scores of available inventory holders 830 withnon-“A” destination items. For example, the priority score of theinventory holder 830 g may be reduced (graphically represented in FIG.11 as a rightward shift to the score position 830 g′) and/or thepriority score of the inventory holder 830 h can be increased(graphically represented in FIG. 11 as a leftward shift to the scoreposition 830 h′). However, in a case in which an inventory holder 830 gwith a downgraded priority score will still be selected (as in FIG. 11),an inventory item from the downgraded inventory holder 830 g may bepicked for a container 802 e on the secondary container holder 806 bwhile the primary container holder 806 a may be moved away from the pickstation 850 without all containers 802 a-d being completely full.

In some embodiments, the discharge container selection module 738 (FIG.9) for a discharge operation (such as at 678 shown in FIG. 8), selects aparticular O-state container holder 606 to be moved to a dischargestation. The selection of the particular O-state container holder 606 tobe moved may be based on an amount of time that the holder 606 has beenin the O-state. The discharge station selected for the particularO-state container holder 606 may be based on a projected time until thedischarge operation. However, selection of the particular O-statecontainer and/or discharge station alternatively or additionally can bebased on other suitable types of priority analysis. FIG. 12 illustratesan example scenario of determining an allocation of container holders todischarge stations. In the example scenario, a first O-state containerholder 906 a was loaded at 4:15 pm, while a second O-state container 906b was loaded at 4:18 pm. The first O-state container holder 906 a, beingolder than the second O-state container holder 906 b, is selected firstfor allocation to a discharge station by the discharge containerselection module 738. For the first O-state container holder 906, thedischarge container selection module 738 evaluates a travel time and await time associated with each available discharge station. For example,the discharge container selection module 738 determines that processingat a first discharge station 950 a will involve 20 seconds of traveltime (e.g., based on a distance from the first O-state container holder906 to the first discharge station 950 a) and zero seconds of wait time(e.g., based on the present queue of zero additional container holders906 at the first discharge station 950 a), while processing at a seconddischarge station 950 b will involve 10 seconds of travel time and 20seconds of wait time (e.g., based on the two additional containerholders 906 c, 906 d that will still be in the queue after the traveltime to the first discharge station 950 a). After comparing the net timeof 20 seconds associated with the first discharge station 950 a againstthe net time of 30 seconds associated with the second discharge station950 b, the discharge container selection module 738 selects the firstdischarge station 950 a for the first container holder 906 a based onthe lower net time. In some embodiments, the discharge containerselection module 738 may determine allocation of container holders 906and discharge stations 950 based on other additional or alternativefactors, including, but not limited to, whether any station 950 willexperience an interruption in operations if a container holder 906 isnot sent to the station 950 or an urgency with which inventory itemsshould be discharged, e.g., to fulfill customer orders.

Other variations are within the spirit of the present disclosure. Thus,while the disclosed techniques are susceptible to various modificationsand alternative constructions, certain illustrated embodiments thereofare shown in the drawings and have been described above in detail. Itshould be understood, however, that there is no intention to limit theinvention to the specific form or forms disclosed, but on the contrary,the intention is to cover all modifications, alternative constructionsand equivalents falling within the spirit and scope of the invention, asdefined in the appended claims.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the disclosed embodiments (especially in thecontext of the following claims) are to be construed to cover both thesingular and the plural, unless otherwise indicated herein or clearlycontradicted by context. The terms “comprising,” “having,” “including,”and “containing” are to be construed as open-ended terms (i.e., meaning“including, but not limited to,”) unless otherwise noted. The term“connect” is to be construed as partly or wholly contained within,attached to, or joined together, even if there is something intervening.Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein and eachseparate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate embodiments of the invention anddoes not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this disclosure are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

All references, including publications, patent applications and patents,cited herein are hereby incorporated by reference to the same extent asif each reference were individually and specifically indicated to beincorporated by reference and were set forth in its entirety herein.

What is claimed is:
 1. A management module configured to manageinventory within a workspace in a facility, the workspace comprising: aplurality of inventory holders configured to store inventory itemswithin the workspace; a plurality of container holders configured tomove inventory items within the workspace; a plurality of containersthat are each i) configured to be carried by, and removable from, atleast one of the container holders; and ii) configured for receiving oneor more inventory items; a plurality of stations within the workspace,each station in the plurality of stations configured to receive at leastone of an inventory holder or a container holder for operationsinvolving the received at least one of the inventory holder or thecontainer holder; and a plurality of unmanned mobile drive units, atleast some of the unmanned mobile drive units configured for moving theinventory holders to and from the stations, and at least some of theunmanned mobile drive units configured for moving the container holdersto and from the stations; the management module further configured to:instruct at least one of the unmanned mobile drive units to move acontainer holder, without any of the containers thereon, to a firststation of the plurality of stations; instruct, at the first station,loading the container holder with a container containing a firstinventory item, thereby setting the container holder to an inboundstate; instruct at least one of the unmanned mobile drive units to movethe container holder in the inbound state to a second station of theplurality of stations; instruct at least one of the unmanned mobiledrive units to move a first inventory holder to the second station;instruct, at the second station, transferring the first inventory itemfrom the container to the first inventory holder so that the containeris emptied, thereby setting the container holder carrying the emptycontainer to an empty state; instruct at least one of the unmannedmobile drive units to move the container holder in the empty state to athird station of the plurality of stations; instruct at least one of theunmanned mobile drive units to move a second inventory holder to thethird station; instruct, at the third station, transferring a secondinventory item from the second inventory holder to the container at thethird station so that the container contains the second inventory item,thereby setting the container holder to an outbound state; instruct atleast one of the unmanned mobile drive units to move the containerholder in the outbound state to a fourth station of the plurality ofstations; and instruct, at the fourth station, unloading the containercontaining the second inventory item from the container holder, therebysetting the container holder, without any of the containers thereon, toan unoccupied state.
 2. The management module of claim 1, wherein themanagement module is configured to instruct at least one of the unmannedmobile drive units to move the container holder to a storage area so asto store the container holder in at least one of the following: theunoccupied state; the inbound state; the empty state; or the outboundstate.
 3. The management module of claim 1, wherein the managementmodule is configured to instruct, at a station, unloading an emptycontainer from the container holder, thereby setting the containerholder to the unoccupied state.
 4. The management module of claim 1,wherein the management module is configured to instruct, at a station,loading the container holder with an empty container, thereby settingthe container holder to the empty state.
 5. A management moduleconfigured to manage inventory within a workspace in a facility, theworkspace comprising: a plurality of inventory holders configured tostore inventory items within the workspace; a plurality of containerholders, each container holder comprising one or more berths; aplurality of containers that are each i) configured to be carried by,and removable from, at least one berth of at least one of the containerholders; and ii) configured for receiving one or more inventory items;an unoccupied subset of the plurality of container holders comprisingcontainer holders each having at least one unoccupied berth without anyof the containers therein; an inbound subset of the plurality ofcontainer holders comprising container holders each having at least oneinbound berth carrying a container of one or more inbound inventoryitems; an emptied subset of the plurality of container holderscomprising container holders each having at least one emptied berthcarrying a container emptied of inventory items; an outbound subset ofthe plurality of container holders comprising container holders eachhaving at least one outbound berth carrying a container of one or moreoutbound inventory items; a plurality of stations within the workspace,each station configured to receive at least one of an inventory holderor a container holder for operations involving the received at least oneof the inventory holder or the container holder; and a plurality ofunmanned mobile drive units, at least some of the unmanned mobile driveunits configured for moving the inventory holders to and from thestations, and at least some of the unmanned mobile drive unitsconfigured for moving the container holders to and from the stations;the management module further configured to: select a first containerholder from the unoccupied subset; instruct at least one of the unmannedmobile drive units to move the first container holder to a firststation; instruct, at the first station, loading an unoccupied berth ofthe first container holder with a first container of one or more inboundinventory items, thereby converting the unoccupied berth to an inboundberth and including the first container holder in the inbound subset;select a second container holder from the inbound subset, the secondcontainer holder having an inbound berth carrying a second container ofone or more inbound inventory items therein; instruct at least one ofthe unmanned mobile drive units to move the second container holder to asecond station; select a first inventory holder; instruct at least oneof the unmanned mobile drive units to move the first inventory holder tothe second station; instruct, at the second station, transferring one ormore of the inbound inventory items from the second container carried bythe inbound berth of the second container holder to the first inventoryholder so that the second container is emptied, thereby converting theinbound berth of the second container to an emptied berth and includingthe second container holder in the emptied subset; select a thirdcontainer holder from the emptied subset, the third container holderhaving an emptied berth carrying an emptied container; instruct at leastone of the unmanned mobile drive units to move the third containerholder to a third station; select a second inventory holder; instruct atleast one of the unmanned mobile drive units to move the secondinventory holder to the third station, the second inventory holdercomprising one or more outbound inventory items; instruct, at the thirdstation, transferring one or more of the outbound inventory items fromthe second inventory holder to the emptied container carried by theemptied berth of the third container holder, thereby converting theemptied berth to an outbound berth and including the third containerholder in the outbound subset; select a fourth container holder from theoutbound subset, the fourth container holder having an outbound berthcarrying an outbound container of one or more outbound inventory itemstherein; instruct at least one of the unmanned mobile drive units tomove the fourth container holder to a fourth station; and instruct, atthe fourth station, unloading the outbound container from the outboundberth of the fourth container holder, thereby converting the outboundberth to an unoccupied berth and including the fourth container holderin the unoccupied subset.
 6. The management module of claim 5, whereinthe management module is further configured to select the firstcontainer holder from the unoccupied subset based at least in part on aproximity of the first container holder to the first station.
 7. Themanagement module of claim 5, wherein the management module is furtherconfigured to select the first container holder from the unoccupiedsubset based at least in part on a utility of a storage space occupiedby the first container holder.
 8. The management module of claim 5,wherein the management module is further configured to select the secondcontainer holder from the inbound subset based at least in part on thesecond container carrying an inbound container that is older than aninbound container of another container holder in the inbound subset. 9.The management module of claim 5, wherein the management module isfurther configured to select the first inventory holder based at leastin part on a proximity of the first inventory holder to the secondstation and an amount of space in the first inventory holder for storingthe one or more inbound inventory items carried by the second containerof the second container holder.
 10. The management module of claim 5,wherein the management module is further configured to select the thirdcontainer holder based at least in part on a proximity of the thirdcontainer holder to the third station.
 11. The management module ofclaim 5, wherein the management module is further configured to selectthe second inventory holder based at least in part on requests for oneor more of the outbound items stored in the second inventory holder. 12.The management module of claim 5, wherein the management module isfurther configured to select the second inventory holder based at leastin part on a proximity of the second inventory holder to the thirdstation.
 13. The management module of claim 5, wherein the managementmodule is further configured to select the second inventory holder basedat least in part on designated destinations of the one or more outboundinventory items.
 14. The management module of claim 5, wherein themanagement module is further configured to select the fourth containerholder based at least in part on a proximity of the fourth containerholder to the fourth station.
 15. The management module of claim 5,wherein the management module is further configured to select the fourthcontainer holder based at least in part on an estimated amount of timeuntil the outbound container will be unloaded from the fourth containerholder at the fourth station.
 16. The management module of claim 15,wherein the estimated amount of time is based at least in part on atravel time of the fourth container holder to reach the fourth stationand a wait time until the fourth station is available to unload thefourth container holder.
 17. A method of managing inventory comprising:instructing, by a management module, at least one unmanned mobile driveunit of a plurality of unmanned mobile drive units to move a firstcontainer holder having an unoccupied berth without any containerstherein to an induct station; instructing, by the management module,loading at the induct station said unoccupied berth of the firstcontainer holder with a first container of one or more inbound inventoryitems, thereby converting said unoccupied berth to an inbound berth andincluding the first container holder in an inbound subset of containerholders; instructing, by the management module, at least one of theunmanned mobile drive units of the plurality of unmanned mobile driveunits to move a second container holder having an inbound berth carryinga second container of one or more inbound inventory items therein to astow station; instructing, by the management module, at least one of theunmanned mobile drive units of the plurality of unmanned mobile driveunits to move a first inventory holder to the stow station; instructing,by the management module, transferring at the stow station one or moreof the inbound inventory items from the second container carried by theinbound berth of the second container holder to the first inventoryholder so that the second container is emptied, thereby converting theinbound berth of the second container holder to an emptied berth andincluding the second container holder in an emptied subset of containerholders; instructing, by the management module, at least one of theunmanned mobile drive units of the plurality of unmanned mobile driveunits to move a third container holder having an emptied berth carryingan emptied container to a pick station; instructing, by the managementmodule, at least one of the unmanned mobile drive units of the pluralityof unmanned mobile drive units to move a second inventory holder to thepick station, the second inventory holder comprising one or moreoutbound inventory items; instructing, by the management module,transferring at the stow station one or more of the outbound inventoryitems from the second inventory holder to the emptied container carriedby the emptied berth of the third container holder, thereby convertingthe emptied berth to an outbound berth and including the third containerholder in an outbound subset of container holders; instructing, by themanagement module, at least one of the unmanned mobile drive units ofthe plurality of unmanned mobile drive units to move a fourth containerholder having an outbound berth carrying an outbound container of one ormore outbound inventory items therein to a discharge station; andinstructing, by the management module, unloading at the dischargestation the outbound container from the outbound berth of the fourthcontainer holder, thereby converting the outbound berth to an unoccupiedberth and including the fourth container holder in an unoccupied subsetof container holders.
 18. The method of claim 17, further comprising:instructing, by the management module, when a station is not availablefor an operation involving a given container holder, at least one of theunmanned mobile drive units of the plurality of unmanned mobile driveunits to move the given container holder to a mixed storage areaconfigured to store, within the workspace, at least some containerholders when not received at stations.
 19. The method of claim 17,further comprising: instructing, by the management module, unloading anemptied container from an emptied berth of a given container holder at astation, thereby converting the emptied berth to an unoccupied berth andincluding the given container holder in the unoccupied subset ofcontainer holders.
 20. The method of claim 17, further comprising:instructing, by the management module, loading an emptied container at astation into an unoccupied berth of a given container holder, therebyconverting the unoccupied berth to an emptied berth and including thegiven container holder in the emptied subset of container holders.